All you need is rice flour and daidai, a Japanese type of bitter orange... Then you make two mochi rice cakes, as big as you want them, but one should be smaller. Then, the smaller one is placed over the other and the daidai is placed in top. Since the two mochi are rouded, they look similar to the copper round mirrors (kagami in Japanese) used during the Muromachi period, hence the name: kagami-mochi.
Lately, I saw many kagamimochi made of plastic - they look good, but... they don't have the same charm... (^_^)
EXIF info:
Nikon D90
Lens: 8-16mm F/4.5-5.6G
Focal Length: 14mm
Aperture: F/5.6
Shutter Speed: 1/60s
ISO Sensitivity: ISO 320